Hi,
If you are using "mpiexec" from OSC there is the -pernode and -npernode options. I use them so that I can run Hybrid(mpi/openmp) parallel codes, 1 mpi process per node and 2 or 4 openmp threads.
the pbs resources request lines like this in my PBS scripts.
#PBS -l walltime=00:10:00
#PBS -l nodes=3:cluster:ppn=2
mpiexec -pernode ./executable
Chris Bording
High Performance Computing Application Analyst
The Virginia Institute of Marine Science
757-221-3488
rcbord at wm.edu